In digital subscriber line (DSL) systems, the same cable is typically used for both transmitting and receiving signals within the network. As such, the receiver not only receives signals from the far end of the cable (such signals are typically attenuated significantly by losses in the cable), but also transmits signals, which are not attenuated by cable losses. For typical DSL systems, signals are received with spectral contents at levels that can be less than −140 dBm/Hz, while signals may be transmitted at levels as high as −40 dBm/Hz. This implies that the DSL system should support a dynamic range in excess of 100 dB, which is particularly challenging in discrete multi tone (DMT) systems as the peak-to-average power ratio (PAR) is approximately 15 dB. This effectively adds another 12 dB on top of the linearity requirement. This increases the dynamic range in excess of 112 dB when measured with a sinusoidal input signal having a PAR of 3 dB.
Linearity and noise requirements of the receiver are generally not driven by the system's ability to receive the signal from the far end as this signal is small and has been significantly attenuated. Rather, the requirements are driven by the noise and echo signal coupling in from the transmitted signal as this signal is much larger as it has not been attenuated by the cable. Most DSL systems rely on FDM (frequency division multiplexing), where the transmitted and received signals are at different frequencies, and where band-pass filtering is used to reduce echo and out of band noise. However, some systems such as symmetric DSL (SHDSL) systems and full overlap echo-cancelled asymmetric DSL (ADSL) systems share receive and transmit frequencies, thereby making filtering impossible. Furthermore, filtering can be expensive from the standpoint of additional bill of material (BOM) if the filtering is implemented with external filters. If the filtering is implemented on-chip with integrated filters, the cost can be expensive from the standpoint of power and silicon area required.